The Surprising Secret of Synchronization

This video explains the problem - it is worth watching the entire video. If you are pressed for time jump to 15:00 mark [VIDEO]

Apparently an incident took place in 1831 where a bridge collapsed under soldiers marching, but the Wikipedia article [1] notes that "The conclusion of the investigation was that the vibration caused by the marching precipitated the bolt's failure, but it would have failed eventually anyway". There's more interesting information on synchronization and bridges on Veritasium's video, The Secret of Synchronization [2]. [1] [LINK] [2] [VIDEO]

There's some synchronization effects in metronomes and in industrial equipment. If you have several things that can oscillate, perhaps not at precisely the same rate but close, and the motions reinforce each other when they do achieve the same rate, you can get surprising synchronization. For an intuitive example, see this Veritasium video, starting at around 4:50: [VIDEO] . This happens in crowds of people, for an example, the London Millenium pedestrian bridge wobble: [LINK] . I was initially curious how this could happen with a heart rate; I could imagine some shared feedback mechanism for walking, or breathing, but a shared feedback for heart rate was surprising. Perhaps if they were touching, and there was some electrical or pressure-sensitive pulse? Disappointing that it's merely the delta between exciting and dull parts of the stories.